Hydroxyapatite Formation during Hydrothermal Liquefaction of Algae: Synthesis and Catalytic Implications

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Abstract Title: Understanding the role of calcium phosphate in improving oil properties during thermochemical conversion of wastewater-fed algae to biocrude Algae have long been investigated as a potential feedstock for renewable, affordable, and reduced carbon-footprint fuels. Hydrothermal liquefaction (HTL) is a process to convert wet algae to a carbon-rich biocrude. HTL processing of algae grown in municipal wastewater has been shown to produce a biocrude with properties similar to conventional petroleum crude. Furthermore, the HTL of wastewater-fed algae results in the production of a solid biochar product, which has hitherto been considered a waste product. However, evidence shows that wastewater-derived biochar contains a form of calcium phosphate, which may act as a catalyst to improve biocrude composition and stability. An award is made to Professors Susan Stagg-Williams and Belinda Sturm of the University of Kansas to understand the fundamental relationships between growth conditions, the formation of biochar during the HTL reaction, the formation of calcium phosphate crystals, and the ultimate impact on biocrude yield and properties. This knowledge is necessary for the ultimate commercialization of algal technology for renewable fuels and chemicals. Various educational programs are planned by the investigators which will increase public awareness and engagement with sustainable energy production. Hydrothermal liquefaction has emerged as a promising whole cell conversion technology utilizing subcritical water to convert algal biomass to a carbon-rich biocrude. Preliminary work at the University of Kansas has demonstrated that HTL of wastewater-cultivated algae leads to high biocrude yields, low biocrude O/C ratios, low levels of high boiling point compounds in the biocrude, and high production of biochar. Characterization studies have shown that the biochar produced is a calcium-deficient substituted hydroxyapatite (HAp) form of calcium phosphate. HAp has been shown to have acid-base properties and be catalytically active for the dehydration and dehydrogenation reactions. In this study, the liquefaction of algae and model compounds will be investigated to determine: 1) the impact of algal growth conditions (Ca, N, P) on the crystallization of HAp in the HTL reactor and the resultant impact on biocrude composition, 2) the impact of the HAp crystallite formation on the macromolecule monomers formed during the liquefaction reaction, and 3) the ability of HAp synthesized during liquefaction to catalytically alter the biocrude composition through dehydration and dehydrogenation reactions. Coupling controlled HAp crystal growth with the production of biocrude is novel. Generating an understanding of the relationship between growth conditions, the formation of HAp, and the ultimate impact on biocrude yield and properties has significant merit for the ultimate commercialization of algal HTL technology. Another aspect of this project supporting the recommendation for award is the environmental focus. Increasing the fundamental knowledge about the application of algal treatment systems for phosphorus removal and recovery from municipal and agricultural wastewater should yield positive environmental and economic impacts. Utilizing algal treatment systems as a low-cost alternative to nutrient removal, while generating biocrude and high-value catalysts and biomaterials, has the potential to turn a historically regulatory operation (wastewater treatment) into a revenue-generating operation.